The present invention relates to a bolometric detector, an imaging system including a bolometric detector and a radiation analyzing system provided with a bolometric detector.
For detecting radiation in the range of 8 to 14 microns (.mu.m) it is known, inter alia, from an article entitled "Miniature Optically Immersed Thermistor Bolometer Arrays" by (R. de Waard and S. Weiner, and published in Applied Optics, August 1967, Vol. 6, No. 8, pages 1327 to 1331), that the optical immersion of a thermistor flake on the plane surface of an optical immersion lens in the form of a germanium hemisphere produces an optical gain of four. That is, the apparent optical size of the thermistor flake is increased by the refractive index times the physical dimensions of the flake. Additionally, in theory the sensitivity of the immersed thermistor flake is approximately four times that of its unimmersed equivalent, since the responsivity of the bolometer increases approximately inversely as the square root of the flake area. However, reflection and absorption losses in the germanium of the hemisphere prevent full realization of this gain and also render the detector response spectrally dependent. A further problem with such a bolometric detector is that the germanium hemisphere acts as a heat sink and consequently the thermistor loses heat and the change in resistance does not relate exactly in the changes in the radiant power incident on the hemispherical surface of the germanium.
It has also been proposed to use optically immersed photoconductive HgCdTe detectors as photon detectors in, for example, a letter to the editor of Infrared Physics ("Optical Immersion of HgCdTe Photoconductive Detectors," Joseph E. Slawek et al, Infrared Physics, 1975, Vol. 15, pages 339 and 340.
Optically immersed photoconductive cells are also disclosed in U.S. Pat. No. 2,964,636.
There has also been an interest in using pyroelectric materials for detecting radiation in the wavelength range of 8 to 14 .mu.m. However the pyroelectric material, which may be an element of 30 .mu.m thickness, is difficult to handle and has a low thermal capacity. If it was mounted directly on a plane surface of a germanium hemisphere then its sensitivity would be affected adversely by the heat lost to the germanium, and if it was spaced from the plane surface of the germanium hemisphere to reduce the heat losses, then reflection losses would occur. Also the element is not easy to mount. Because of these problems, as far as is known a bolometric detector using a pyroelectric detector element has not been made so far.